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One rule of life: Are we poised on the border of order?

By Philip Ball

There are signs that all living things sit on the knife-edge of criticality – something that could help them adapt to complex and unpredictable events

WHEN physicists take an interest in the living world, some biologists fear the worst. After all, goes the bad joke, there’s only so much you can gain by modelling a cow as a sphere. But one crucial idea from physics may hold valuable insights into complex biological behaviour in everything from birds to gene networks. There is increasing evidence that many systems we observe in living things are close to what’s called a critical point – they sit on a knife-edge, precariously poised between order and disorder. Odd as it may sound, this strategy could confer a variety of benefits, in particular the flexibility to deal with a complex and unpredictable environment.

Some of the most convincing evidence comes from neuroscience. For a decade now, we have been seeing clues that neurons in the brain sit near a critical point. On one side, they are stable and ready to respond to stimuli. On the other, they fire in an uncontrolled cascade, triggering a seizure. Neuroscientists believe that being poised in between can help explain basic aspects of the brain’s functions.

Similar behaviour has been spotted in bird flocks, insect swarms and even inside cells. This proximity to “criticality” could help creatures adapt rapidly, says physicist Jim Sethna of Cornell University in Ithaca, New York. “At the critical point, everything is about to go crazy. You get massively more sensitive behaviour.” It is, he says, like having some general-purpose knobs that living things can adjust …